Electrowetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction the basic principles of the technology” can be found in (“Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
U.S. Pat. No. 6,565,727 (A. Shenderov; issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al, issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
EWOD devices have been identified as a promising platform for Lab-on-a-chip (LoaC) technology. LoaC technology is concerned with devices which seek to integrate a number of chemical or biochemical laboratory functions onto a single microscopic device. There exists a broad range of potential applications of this technology in areas such as healthcare, energy and material synthesis. Examples include bodily fluid analysis for point-of-care diagnostics, drug synthesis, proteomics, etc.
Thin film electronics based on thin film transistors (TFTs) is a very well known technology which can be used, for example, in controlling Liquid Crystal (LC) displays.
Many modern displays use an Active Matrix (AM) arrangement whereby a switch transistor is provided in each pixel of the display. Such displays often also incorporate integrated driver circuits to supply voltage pulses to the row and column lines (and thus program voltages to the pixels in an array). These are realised in thin film electronics and integrated onto the TFT substrate.
U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies. Such an approach may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD).
When performing droplet operations it is in general very useful to have some means of sensing droplet position, size and constitution. This can be implemented by a number of means. For example an optical means of sensing may be implemented by observing droplet positions using a microscope. A method of optical detection using LEDs and photo-sensors attached to the EWOD substrate is described in Lab Chip, 2004, 4, 310-315. U.S. Pat. No. 7,163,612 (J. Sterling et al.; issued Jan. 16, 2007) noted above also describes how TFT-based sensor circuits may be used with an AM-EWOD, e.g. to determine drop position. In the arrangement described there are two TFT substrates, the lower one being used to control the EWOD voltages, and the top substrate being used to perform a sensor function.
Sensors and Actuators B, Vol. 98 (2004) pages 319-327 describes a method for measuring droplet impedance by connecting external PCB electronics to an electrode in an EWOD array. However a disadvantage of this method is that the number of array elements at which impedance can be sensed is limited by the number of connections that can be supplied to the device. Furthermore this is not an integrated solution with external sensor electronics being required. This paper also describes how measured impedance can be used to meter the size of droplets and how droplet metering can be used to accurately control the quantities of reagents of chemical or biochemical reactions performed using an EWOD device. Impedance measurements at one or more locations could also be used for any of the following:                Monitor the position of droplets within an array.        Determining the position of droplets within the array as a means of verifying the correct implementation of any of the previously droplet operations.        Measuring droplet impedance to determine information regarding drop constitution, e.g. conductivity.        Measuring droplet impedance characteristics to detect or quantify a chemical or biochemical reaction.        
It is also known that optical means of sensing can be implemented onto a TFT substrate, for example as described in “A Continuous Grain Silicon System LCD with Optical Input Function”, Brown et al. IEEE Journal of Solid State Circuits, Vol. 42, Issue 12, December 2007 pp 2904-2912. The same reference also describes how sensor driver circuits and output amplifiers for the readout of sensor data can also be integrated onto the same TFT substrate.
Other modes of sensing integrated within a TFT substrate are also known. For example WO 2008/117210 (D. Fish et al.; published Oct. 2, 2008) describes a means of integrating thermal sensors in a TFT substrate.
Digital microfluidics can also be implemented using technologies other than EWOD. For example dielectrophoresis is a technique which may be used, as described in Thomas P Hunt et al, Lab Chip, 2008, 8, 81-87 which describes a silicon integrated circuit (IC) backplane to drive a dielectrophoresis array for digital microfluidics.
In the prior art systems described above, it is possible to implement feedback between a sensor function and a droplet control function. Such systems require some external means of processing sensor data in order to determine the subsequent control data to be written to the device. The provision of such external means (e.g. additional electronic circuitry, a computer etc) has disadvantages in that it adds cost and complexity to the overall system and may result in increased overall system power consumption. A further disadvantage is that feedback may be slower, since time is required to read-out and process sensor data, and determine and write the subsequent control signals.